This invention relates generally to color electrophotography, and more specifically to the use of a series of toners which are prepared by blending different amounts of the subtractive primary toners in order to obtain a choice of colors using a single pass electrophotographic system, preferably a xerographic system.
The formation and development of images in an electrophotographic system, and more specifically a xerographic system is well known as described, for example, in U.S. Pat. No. 2,297,691. In such systems, several methods are used for applying the electroscopic particles or toner to the latent electrostatic image to be developed, such methods including cascade development, U.S. Pat. No. 2,618,552; magnetic brush development, U.S. Pat. No. 2,874,063; powder cloud development, U.S. Pat. No. 2,221,776; touchdown development, U.S. Pat. No. 3,166,432 and the like. Generally, the toners used in these types of systems result in the production of black images.
Recently there has been developed processes and materials for use in the formation of color images. Electrophotographic color systems are generally based on trichromatic color synthesis, such as the substractive color formation types. Accordingly, in electrophotographic color systems, toner or developing particles of at least three different colors are employed to synthesize any desired color. Generally, at least three color separation images are formed and the combined images register with each other to form a colored reproduction of a full colored original. In color xerography as described, for example, in Dessauer U.S. Pat. No. 2,962,374, at least three latent electrostatic images are formed by exposing a xerographic plate to different optical color separation images. Each of the latent electrostatic images is developed with a different color toner and subsequently the three toner images are combined to form the final full color image. This combination of three color toner images is generally made on a copy sheet such as paper to which the toner images are permanently affixed. One of the most common techniques for fixing these toner images to the paper copy sheet is by employing a resin toner which includes a colorant, and heat fixing the toner images to this copy sheet. Images may also be fixed by other techniques such as, for example, subjecting them to a solvent vapor.
In one known process an electrostatic latent image is exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photoreceptor. This electrostatic latent image is then developed with the complimentary magenta color toner to form a magenta colored image corresponding to said electrostatic latent image and transferred in register to an image receiving member. The photoreceptor is then electrostatically charged uniformly in the dark and exposed through a red filter to an imagewise projection of a color image in register with said magenta developed image to form a second electrostatic latent image which second image is developed with the complimentary cyan-colored toner and likewise transferred in register. The photoconductor is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a color image in register with said magenta and cyan developed images to form a third electrostatic latent image which is then developed with the complimentary yellow toner and again transferred in register. The sequence of exposures through colored filters in this multiple development process may be performed in any suitable sequence other than the green, red and blue mentioned.
In these systems one important aspect resides in registration of the color toner image on the copy sheet, that is, the cyan, magenta, and yellow image should be in registration on the receiving member.
Generally, each developer used comprises a toner or resin colored mixture in combination with an appropriate carrier. The toners used must possess the appropriate color and continue to function under machine conditions which expose the developer to impaction and humidity among other undesirable factors. A three color system that has been well known and used in the past includes pigments of suitable cyan, magenta and yellow materials. One of the problems associated with the prior art processes is that it is necessary to use multiple passes, that is, three steps in development with three different colors, which can become cumbersome, uneconomical and slow. Other disadvantages of the prior art processes include the requirements that, (1) the photoreceptor be panchromatic, (2) the development response of each of the three toner developers be constant with usage, and (3) the transfer of the three different developed images be constant.
Also it is known in the prior art that the three color layers can be coated one on top of the other, the first layer being the magenta layer, the second being the cyan layer and the third being the yellow layer. Each subtractive color transmits two thirds of the spectrum and absorbs one third. The combination of cyan, magenta and yellow layers appears black, while the combination of magenta and yellow layers appears red, the combination of magenta and cyan layers appears blue and the combination of yellow and cyan layers appears green.